The present invention relates to optical systems and, in particular, to adjustable apertures for camera lenses.
Since the early days of photography, camera manufacturers have given photographers access to critical adjustments in order to get correctly exposed pictures. These adjustments are commonly referred to by photographers as “shutter speed” (adjustable exposure time of the film), “film speed” (choice of film sensitivity), and lens aperture (adjustable diaphragm in the lens). In addition to affecting the film exposure, these adjustments also provide other essential benefits. For example, the shutter speed adjustment allows the photographer to freeze in time a fast moving scene. The film speed allows the photographer to get the desired grain in the image. The lens aperture adjustment allows the photographer to get the desired depth of field.
In digital cameras, the electronic shutter control (adjustable integration time of the image sensor) often replaces the mechanical shutter but does not eliminate the need for the lens aperture adjustment. Although the correct exposure can be achieved by adjusting the electronic shutter alone, the depth of field cannot be affected by it. Therefore the lens aperture adjustment remains an indispensable tool, not only to control the amount of light impinging on the imaging sensor but also to achieve the desired depth of field. The most common form of lens aperture adjustment is the mechanical iris diaphragm or mechanical iris. The mechanical iris consists of multiple blades which can be moved with respect to each other so as to form a pseudo-circular polygonal aperture; the larger the number of blades, the closer the polygonal aperture is to circular. The blades are often attached to an inner ring and an outer ring. The mechanical iris is opened and closed by turning the outer ring while holding the inner ring stationary. It is sometimes combined with a shutter mechanism. Lenses with a manual iris often bear markings corresponding to the diameter of the iris as a fraction of the lens focal length. This is commonly known as the f-number or f stop, e.g. f/5.6. FIG. 1 shows an example of an 8-blade mechanical iris with apertures ranging from f/1.4 to f/22.
Most film cameras and many digital cameras incorporate a mechanical iris or some other form of lens aperture adjustment (e.g., a rudimentary aperture wheel). However, there are some notable exceptions: disposable film cameras and very-low-cost digital cameras. The main reason for not using a lens aperture adjustment is cost. That is, mechanical irises can be as inexpensive as $1.50, which is very affordable in the design of a $300 digital camera but prohibitively expensive for a $4.50 camera module intended for computer or cellular telephone applications. In fact, almost all cellular telephone cameras (referred to in the industry as cell phone camera modules) do not include a lens aperture adjustment. Originally designed as gadgets rather than replacements for traditional cameras, cell phone camera modules were supposed to produce acceptable images in dim light conditions without a flash (e.g., inside a Karaoke bar). For this reason, they were fitted with lenses with a large fixed aperture (e.g., f/2.8, to maximize sensitivity at the expense of the depth of field), and relied on the electronic shutter to adjust the exposure level. The consequence was that they neither produced good quality images at low-light level (objectionable shot noise and readout noise) nor at high-light level (poor depth of field and reduced sharpness due to lens aberrations).
Because of their enormous popularity (already outselling film and digital cameras), cell phone camera modules are now poised to replace traditional cameras. They need however to match traditional camera image quality at a fraction of the cost of a traditional camera. As unrealistic as it may seem, consumer expectation is that image quality from a $4.50 cell phone camera module should match the image quality from a $300 digital camera. Unfortunately, current cell phone camera modules are optimized for worst-case conditions (low light level imaging) and do not produce images with sufficient sharpness and depth of field, even under good illumination conditions such as outdoor imaging with adequate daylight.
This issue is further aggravated by price pressure and market demand for a larger number of pixels. As semiconductor technology progresses, image sensors get sharper (0.25 megapixels in 2000, 2 megapixels in 2006) and pixels get smaller (5 μm in 2000, 2 μm in 2006) thus requiring a lens with a wider aperture in order to maintain the same sensitivity. This requirement conflicts with the need for a sharper lens (since a wider aperture results in greater lens aberrations) and for an increased depth of field (since a wider aperture results in a reduced depth of field). Rather than solving the fundamental issue at hand (i.e. the need for an adjustable lens aperture), many lens and module manufacturers (as well as start-up companies) have spent millions of dollars trying to circumvent it. See, for example, the Oct. 2, 2006, Red Herring article entitled “Clearer Vision,” the entire disclosure of which is incorporated herein by reference for all purposes.
The two most advertised “band-aid” solutions are the optical auto-focus using a “liquid lens” and the “phase-mask” approach using image processing algorithms. In the case of the optical auto-focus using a liquid lens, the depth of field is not increased. Rather, the focus is simply adjusted for a particular distance. In the case of the phase-mask approach, the focus of the lens is in fact degraded. A phase-mask (placed on one of the lens elements) introduces a relatively constant amount of defocus throughout an extended depth of field. The sharpness is then partially restored by digital means using image processing algorithms. Unfortunately, the sharpness restoration algorithms also introduce a significant amount of noise in the image.
It is clear that none of these solutions really eliminate the need for an adjustable lens aperture but there are no suitable technical implementations fulfilling this need. Current mechanical irises are too expensive, too bulky, too fragile, and too power-hungry to satisfy the expected one-billion cell phone camera module market. Mechanical irises also have another serious technical drawback: diffraction through their circular aperture significantly degrades the image sharpness for small aperture settings, e.g., high f numbers such as f/5.6 or higher.